Process for producing chrome steels and a converter for carrying out the process

ABSTRACT

Process of producing chrome steels of a chromium content of about between 16 to 30 percent and low carbon and nitrogen contents. According to the process, a pig iron charge having a high chromium content and a carbon content of from 2 to 7 percent is refined by blowing with very pure oxygen under reduced pressure conditions, the oxygen being introduced into the melt below the level of the melt. The process is applicable to both ferritic and austenitic chrome steel production. The application also discloses a novel bottom-blown converter construction for carrying out the refining procedure.

[ 5] Mar. 12, 1974 United States Patent 91 Kniippel et al.

[ PROCESS FOR PRODUCING CHROME 3,706,549 12/1972 Knuppel et al. STEELS AND A CONVERTER FOR 2,265,511 12/1941 Brassert........................,.......

84,336 11/1868 Absterdam.... CARRYING OUT THE PROCESS 3,325,160 6/1967 Eklund............. [75] Inventors: Helmut Kniippel; Karl Brotzmann, 3.330.645 7/ 967 Moustier et 211...

both of Sul bach R enberg Hutte; 3,372,023 3/1968 Krainer El Hans Georg Fassbinder 3,501,290 3/1970 Finkl et sulzbach'Rmnberg FOREIGN PATENTS OR APPLICATIONS V 4 3 6 6 2 n a n 6 Z .n w s 4 6 9 H 9 7 Q 8 3 B a m m m. e c y m n e a w m n r e .m G E D h s S A l 3 7 Maximitianshutte m.b.l-l., Sulzbackkosenberg Hume Primary ExammerGerald A. Dost Germany or Firm-Lawrence 1. Field Attorney, Agent,

[22] Filed: Nov. 2, 1972 ABSTRACT Appl. No.: 303,161

Related Application Data Process of producing chrome steels of a chromium Division of Ser. No. 116,447, Feb. 18, 1971.

content of about between 16 to 30 percent and low carbon and nitrogen contents. According to the pro- [30] Foreign Application Priority Data cess, a pig iron charge having a high chromium con- Feb. 18, 1970 Germany............................

tent and a carbon content of from 2 to 7 percent is refined by blowing with very pure oxygen under reduced pressure conditions, the oxygen being introduced into the melt below the level of the melt. The process is applicable'to both ferritic and austenitic chrome steel production.

vms

1 BW T 1 2 C 7 6 5/60 The application also discloses a novel bottom-blown converter construction for carrying out the refining procedure.

References Cited I UNITED STATES PATENTS 2.182.064 266/34 V 5 Claims, 1 Drawing Figure PROCESS FOR PRODUCING CHROME STEELS AND A CONVERTER FOR CARRYING OUT THE PROCESS This application is a division of application Ser. No. 116,447 filed Feb. 18, 1971.

FIELD OF INVENTION The invention is concerned with a process for producing chrome steels of a chromium content of 16-30 percent and having low carbon and nitrogen contents.

While the invention is particularly applicable to the production of ferritic chrome steels of the indicated kind and will thus, in the following, primarily be described in connection with ferritic chrome steels, the invention can also be used for the production of austenitic steels.

BACKGROUND INFORMATION AND PRIOR ART Ferritic chrome steels having a chromium content of between l-30 percent and extremely low carbon and nitrogen contents exhibit superior forming properties and ductility. Further, such steels have excellent corrosion resistance and exhibit superior weldability, particularly if the steels are alloyed with molybdenum. In order to obtain steels with the indicated characteristics, the sum total of the carbon and nitrogen contents should be below 0.015 percent or should at least not exceed that value to any significant extent.

It is common practice to remove the carbon from the heat or melt by refining with oxygen. This, however, results in considerable difficulties since in the oxygen refining of chromium-containing melts, the chromium tends to be oxidized while, on the other hand, the nitrogen content of the melt is not sufficiently decreased. The reason for the oxidation of the chromium during the oxygen refining and the high chromium losses resulting therefrom resides in the high oxygen affinity ofthe chromium. The desired lowering of the nitrogen content is not achieved because the melt absorbs nitrogen from the'refining gas and the atmosphere. Further, as is well known in this art, chromium increases the solubility of nitrogen so that the high chromium content in the melt is apt to raise the nitrogen content. In top blowing oxygen refining processes, wherein the oxygen is thus supplied from the top, the increase in nitrogen is also due to the fact that the oxygen jet or stream which emanates from the blowing or jetting lance at high speed results in a suction action, so that air or atmospheric nitrogen is sucked into the converter, thus introducing more nitrogen into the melt.

Numerous processes have been proposed for the purpose of producing chrome steels. All these prior art processes, however, require considerable technical expenditure or result in chrome steels which do not meet the requirements in respect to low carbon and nitrogen contents necessary for imparting to the steels the desired beneficial characteristics referred to.

procedure results in a reduction of the chromium losses caused by chromium oxidation. However, the necessary extremely low carbon content required in ferritic chrome steels cannot be obtained. Furthermore, in this prior art procedure no measures are taken to prevent nitrogen pickup by the melt.

Another prior art process is disclosed in U.S. Pat. No. 2,993,780, which concerns'a procedure for the production of low alloy steels. In this prior art procedure, a pig iron melt is first refined in customary manner to form steel, for example, according to the top blowing oxygen procedure, and after the slag removal, the depth of the bath is decreased by tilting the converter. Subatmospheric pressure conditions are at the same time applied and an inert gas is blown through the melt. This prior art procedure, however, is not concerned with the production of ferritic chrome steels and does not suggest any means for the purpose of achieving the extremely low carbon and nitrogen contents that are desired in ferritic chrome steels. This holds true although the carbon content of the melt is decreased during the blowing of inert gas, such as argon, under vacuumconditions.

A further process for decarburizing steel melts containing about 3-30 percent chromium is disclosed in U.S. Pat. No. 3,046,107. This process suggests that a decreasing amount of oxygen is introduced into the melt together with an inert gas at temperatures of about 1,4001,900 C. The purpose of the introduction of the oxygen and the inert gas is to reduce the chromium loss and to achieve a final carbon content of 0.07 percent. However, this process does not consider the nitrogen content of the steel, and the starting point for the process is a remelt or steel charge.

With a view to producing stainless steels with low carbon content and low nitrogen content, it has more recently been proposed to operate in an arc furnace. The procedure referred to thus suggests to melt in an arc furnace an iron charge containing up to 26 percent of chromium, up to 1 percent of carbon and up to 1 percent of silicon. The melt is subsequently,,and in a separate reaction vessel, decarburized by blowing a mixture of oxygen and argon until the carbon content has been reduced to 0.010 percent.

All the prior art processes, including the ones re ferred to, have as primary purpose to keep as low as possible the chromium losses which occur during the decarburization of the chrome containing steels. This is achieved, on the one hand, by refining at relatively high temperatures and, on the other hand, by the use of a mixed gas consisting of oxygen and an inert gas, at least during a portion of the refining procedure. Further, in these prior art processes, the chromium, which unavoidably is oxidized during the blowing, is recovered for the melt by the addition of reducing agents such as, for example, silicon and/or aluminum. However, the nitrogen content has not been considered of particular importance in these processes, so that the nitrogen content of the customary chrome steels, dependent on the chromium content, usually is between 0.02 and 0.085 percent. For example, a chrome steel containing 9-1 1 percent of chromium, 2-8 percent of manganese, and 1-5 percent of nickel may customarily contain 0.07-0.08 percent nitrogen.

procedure which can be readily carried out on a large technical scale in economical manner and which results in the production of chrome steels, particularly ferritic chrome steels, of exceedingly low nitrogen content, thereby obtaining a chrome steel which has the desired superior characteristics in respect to ductility and shaping or forming.

[t is also an object of the invention to provide a process of the indicated kind which can be carried out with a minimum of expenditure and which results in a chrome steel of superior characteristics.

Generally it is an object of the invention to improve on the art of making chrome steels, both of ferritic and austenitic nature, as presently practiced.

Further, it is an object of the invention to provide a converter structure eminently suitable for carrying out the inventive procedure.

The solution of the inventive tasks is based on the realization that the desired low nitrogen content can be obtained in an economically feasible manner if the composition of the starting melt is chosen such, and the refining procedure is controlled in such a manner, that already during the decarburization with pure oxygen the nitrogen content of the melt is decreased to an extent which is far greater than the customary nitrogen removal which occurs as the result of blowing of pure oxygen.

Briefly, the invention provides that a pig iron or hot metal charge having a high chromium content and a carbon content of about 2-7 percent is refined under reduced pressure conditions by blowing very pure oxygen below the level of the melt.

The success of the inventive procedure is based on the fact that, due to the blowing with very pure oxygen under reduced pressure conditions, a very rapid decarburization is obtained resulting in a carbon content of extremely low values while, at the same time, nitrogen pickup from the refining gas and/or the ambient atmosphere is successfully prevented.

Of essential importance is the unusually high carbon content of the starting melt which leads to an intense CO formation during the refining. This intense CO formation is the cause for the considerable reduction of the nitrogen content already during the first refining phase. In this manner, nitrogen contents can be obtained which are considerably below the nitrogen values of a steel of comparable composition which has been refined in customary manner and subsequently has been degasified in a vacuum.

The carbon content of the starting melt is advantageously made dependent on the nitrogen content of the starting melt. For example, with a nitrogen content of 0.035 percent and at a pressure in the refining vessel of about 0.1 atm, the carbon content should initially be about 6 percent. Experiments have indicated in this context that the pressure above the melt during the refining should be the lower the lower the carbon content is of the original charge. Within a range of 2-6 percent carbon, it may be stated as a rule of thumb that the ratio of initial carbon content to absolute pressure which is 60 in the above example always be maintained. It is therefore recommended, considering an economical performance of the procedure, to start with a relatively high initial carbon content.

Considering that an extremely low nitrogen content is desired, the refining is advantageously performed with oxygen of a purity degree of 99.9 percent which,

in addition to noble gases, may possibly contain not more than 10 ppm of nitrogen. The purity of the argon should be at least 99.95 and preferably 99.99 percent.

Practical experience has indicated that the carbon content, in spite of the high initial value, may be lowered to 0.5 percent while the nitrogen content is at the same time decreased from about 0.035 to 0.010 percent. The decrease in carbon content as result of the refining, despite the intial high carbon content of the melt, does not result in any substantial chromium oxidation. Considering that a rapid decrease of the nitrogen and carbon contents is desired while, at the same time, avoiding substantial chromium slagging, the additional refining is advantageously performed with a mixture of oxygen and an inert gas which is essentially free of nitrogen. Such inert gas may, for example, be argon and/or hydrogen which facilitates the formation of CO bubbles in the melt. The proportion of inert gas in the oxygen may be gradually increased until the inert gas, at the end stage of the blowing, amounts to -400 percent.

The inventive procedure may be carried out particularly economically and advantageously in a bottom blown converter.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawing and descriptive matter in which there is illustrated and described a preferred embodiment of the invention.

The single FIGURE of the drawings indicates a sectional view of the preferred embodiment of the converter suitable for carrying out the inventive procedure.

Referring now to the drawing, the converter shown therein consists of a steel shell 3 which is lined with a refractory lining 4. A bottom plate 5 is provided which supports the removable tuyere bottom 6. One half of the bottom 6, the lefthand side in the drawing, is traversed by a plurality of tuyeres or nozzles 7. Each nozzle consists of an inner tube 9 and an outer tube 1 l, the inner tubes 9 being all connected to a common oxygen line 8, while all the outer tubes 11 are connected to a common inert gas conduit or line 10. It will be noted that the nozzles or tuyeres are inclined relative to the longitudinal axis of the converter.

The mouth 12 of the converter is relatively small and is situated within a hood 13 which is placed on the converter and connected therewith in gas-tight manner. The hood is provided with a vacuum tube 14 and com municates with a container 15 for receiving the alloying ingredients which, after openingof the slide valve 16, are charged from the container 15, without pressure decrease, into the converter. The container 15 with the slide 16 thus acts as a vacuum lock or sluice and the detachable hood 13 is connected with the converter shell in vacuum-tight manner. The converter shell 3 is preferably welded in vacuum-tight manner.

The converter is provided with a pouring hole 17 which, during the refining procedure, is closed by means of a closure or cap 18 in gas-tight manner. Contrary to the customary substantially cylindrically constructed converters, the converter here shown and particularly suitable for the inventive procedure is shaped approximating that of a sphere or ball. Such sphere-like converters are more suitable for the present invention.

'Further, the filling degree of the converter should be lower than in the customary procedures. Thus, for example, if a 20-ton charge is customarily refined in a converter defining a space of m then a converter to be used for refining a -ton charge in accordance with the inventive procedure should have about twice the volume, to wit, about m. It is also important that the leak losses caused by untight areas should not exceed 0.01 percent of the off-gases which are sucked off through the vacuum pipe 14. y

In top blowing oxygen procedures difficulties occur due to the high temperature stresses to which the lance is subjected by the hood. These difficulties are successfully avoided in bottom blown converters. Further, in the instant bottom blown converter, the high degree of utilization of the blown oxygen and the negligible production of brown smoke make it possible to maintain the sub-atmospheric pressure conditions in the converter and the removal by suction of the reaction gases bymeans of a steam ejector pump, which does not require any preceding purification of the off-gases and which at the same time effectively washes out the dust components of the offgases. The relatively small amounts of off-gases can be readily managed and may be discharged through an off-gas line of relatively small inner diameter of about 500 mm.

The invention will now be described by an example, it being understood, however, that this example is being given by way of illustration and not by way of limitation and that many changes may be effected without affecting in any way the scope and spirit of the appended claims.

Example This test was carried out in a converter of the kind shown in the drawing, having an interior space of 30 m The converter was charged with 20 tons of an alloyed pig iron charge containing 22 percent of chro mium, 6 percent of carbon and 0.035 percent of nitrogen. The temperature was 1,450 C and blowing was effected for 20 minutes with 50 Nm/min. of oxygen under'simultaneous blowing of 3 percent of propane, calculated on the amount of blown oxygen. The propane was introduced through the annular space between the inner tubes and the associated outer tubes of the tuyeres. The carbon content of the melt amounted to 0.5 percent while the nitrogen content amounted to 0.010 percent after. this first refining stage. The bath or melt temperature rose during this stage to 1,700 C. The melt could be replenished with about 4 percent of chrome ore, calculated on the weight of the melt. This was done by opening the slide 16 so that the chrome ore dropped into the converter from the container 15. The carbon removal and nitrogen removal was then continued by blowing an oxygenargon mixture. During a further blowing period of 20 min., the argon portion which originally was 10 percent, when the carbon content was 05 percent, was gradually increased to 95 percent, while the carbon content in the melt decreased to 0.005 percent, and the pressure in the final phase of the refining was lowered to 10-20 mmHg. The propane supply was interrupted when the argon moiety of the blowing gas amounted to 50 percent and instead pure argon was used as shell gas. No significant wear of the tuyeres could be observed. The particular advantage of blowing with pure argon as shell gas during the final phase of the refining resides in the fact that the refined steel is characterized by an extremely low hydrogen content. Although during the second blowing phase the amount of theblown gas is decreased to 20 Nm/min a final carbon content of 0.005 percent and a final nitrogen content of 0.008 percent were obtained. With a view topreventing nitrogen pickup, both the emptying of the converter and the pouring of the steel should be effected under protective gas atmosphere.

The starting chromium content of the melt may be lower than would be required in the light of the desired final chromium content. This is so because the high blowing temperature at the end of the. first blowing stage renders it possible to add significant amounts of chromium ore at that time. Considering the significant increase of the solubility of nitrogen with increasing chromium content, the initial relatively low chromium content results correspondingly in a lower initial nitrogen content of the charge to be refined. In order to ex pedite and accelerate the reduction of the chromium ore, silicon and/or aluminimum may at the same time be added to the converter as reduction agent.

According to a modification of the inventive procedure, the refining gas and/or the mixture of refining gas and inert gas may be admixed with lime in powder form. This results, surprisingly, in a particularly quiet blowing without the formation of foaming slag and metal splashing. This general advantage has a particularly favorable effect in the inventive converter construction since the problems caused by the sealing of the structure can be more readily solved. Further, due to the addition of lime, a satisfactory and early phosphorus and sulfur removal takes place. Accordingly, it is then also possible to use less expensive starting materials. If the initial phosphorus and sulfur contents are high, it may be required to operate with two slags. in

experimental melts, the first slag was removed after about 3-10 min. and the refining procedure corresponding to the above example was continued and terminated under a second slag.

As previously stated, the invention is particularly suitable for the production of ferritic steels. However, austenitic steels may also produced in accordance with the inventive procedure and in the inventive converter structure. However, in producing austenitic steels the requirements of the individual process steps need not be exactly adhered to. Thus, for example, it is sufficient if the pressure above the melt is in a range of 1-0.3 atm. The admixture of inert gas during the refining process may be increased toward the end. of the procedure up to percent.

What is claimed is:

l. A convertor structure suitable for the production of chrome steels having a chromium content of about 16-30 percent and having low carbon and nitrogen contents, which comprises:

a convertor steel shell having an open mouth portion and a bottom portion;

a refractory lining said shell;

nozzle means traversing said bottom portion said nozzle means consisting of a plurality of nozzles each nozzle comprising two concentric coaxial tubes;

and a detachable hood member mounted above and enclosing said open mouth portion and being conmon gas supply line.

4. A converter structure as claimed in claim 3, wherein the common gas supply line for said inner tubes is a line adapted to carry oxygen and/or inert gas while the common gas supply line for said outer tubes is a line adapted to carry propane and/or inert gas.

5. A converter structure as claimed in claim 1, wherein said shell has a configuration approximating a sphere. 

2. A converter structure as claimed in claim 1, wherein said nozzle means are inclined relative to the longitudinal axis of said converter.
 3. A converter structure as claimed in claim 1, wherein the inner ones of the concentric tubes are connected to a common gas supply line while the outer ones of said concentric tubes are connected to a common gas supply line.
 4. A converter structure as claimed in claim 3, wherein the common gas supply line for said inner tubes is a line adapted to carry oxygen and/or inert gas while the common gas supply line for said outer tubes is a line adapted to carry propane and/or inert gas.
 5. A converter structure as claimed in claim 1, wherein said shell has a configuration approximating a sphere. 